Charles and Camilla in Trinidad, with the luxurious Leander behind them

Clarence House has openly insisted this was the motive for selecting the Leander for their cruise to Tobago, St Lucia, Montserrat and Jamaica.

Staff working for the Prince, who keenly promotes environmental issues, worked out that using the yacht and schedule flights would drastically reduce carbon emissions from the trip.

According to their calculations, they could be cut by 40 per cent compared with Charles's last royal visit to the Caribbean eight years ago.

Camilla was even instructed to take the Gatwick Express shuttle train to the airport rather than travel by private car in a bid to up the "green" credentials.

Clarence House also argued that the taxpayer would benefit, because it has negotiated a special deal to hire the yacht, which usually costs £280,000 a week to charter.

The slight flaw in the plan is that experts are now claiming that the tour's carbon footprint will actually be more damaging to the environment than hundreds of transatlantic flights.

Scroll down for more...

The Duchess poses alongside a performer at the Staubles Bay coastguard facility. Clarence House says the trip will 'reinforce Britain's ties' with the Commonwealth countries

Charles, 59, and Camilla, 60, will cover 1,5000 miles during their trip, at a modest rate of around 15 knots.

Experts have predicted its fuel consumption will hit 75,000 litres at this pace.

The National Energy Foundation website claims the cruise, in total, will pump out 200 tons of carbon dioxide into the atmosphere.

This is enough to fly the average passenger from London to New York a staggering 260 times.

A Clarence House spokeswoman insisted last night that the figure was based on an unfair comparison and that the carbon emission for the yacht was only slightly more than that emitted by an entire jumbo jet on one single journey to New York.

"The carbon emissions generated by this are considerably less than the other option, which is to charter a plane around the islands.

"We took at all the factors into consideration when we planned the trip. It is also much cheaper and less cost to the taxpayer."

Camilla shades herself from the Caribbean sun at the start of the royal tour

But claims of a "green wash" refuse to disappear, amid rumours that the real reason for using the Leander is Camilla's fear of flying.

Tony Cottee, of the radical direct action environmental pressure group Rising Tide, claiming it was "typical" of the Prince to stress his environmental credentials while continuing to damage the environment.

He said: "He seems to be redefining the dictionary definition of sustainability. It is typical of people in his position.

"They have no basis of understanding who lives in a council flat and is trying to cut their emissions, by example, by buying more efficient white goods, which are still more expensive than inefficient ones.

"When you compare this to people who are struggling to make a difference, somethign like this will blow anything they can do right out of the water for the rest of their lives."

Ironically, the couple will visit a rainforest conservation project during their tour, as well as see some of the bio-diversity in the region.

They will also be going to Kingston's famous Bob Marley Museum and the home of reggae.

The Prince's spokeswoman says the couple's visit will "reinforce Britain's ties with the important Commonwealth countries".

But it has already proved to be the latest opportunity for his critics to question his full commitment to environmental issues.

Charles has previously come under fire for not compromising his lavish lifestyle while insisting on the urgency to act now to combat climate change.